You laid it out well Luka.
And incidentally, part of the reason why I tend to put raw +/- forward as a starting point to conversations is because RAPM isn't a single stat. If there was only one way to do it, and thus only one set of numbers to work with, it would be easier to introduce to people.
I do think everybody should be looked at RAPM - nbashotcharts.com is my current go-to for it - but since it's hard to use it as a first step analysis (due to disagreement by source) and it's not the last step in analysis (because that's holistic), it makes it a less useful tool for dialogue than I once had hoped.
Also, I like your description of the RPM issues. I remember when Englemann first came out with XRAPM (the prototype for what RPM became) and explained it on the APBRmetrics forum around 2012. The response from most of the statisticians was fawning, and meanwhile I was shouting from the rafters:
It's not your job to make an all-in-one stat, it's your job to make something analysts can use in conjunction with the stats that already exist. You've just taken a more useful stat (RAPM) and replaced it with a black-box stat (XRAPM) which factors those other stats in without allowing us to see how it does it, and so we can't even hope to figure out double-counting issues. How the hell do you not see the problem here?
And of course the answer is, these statisticians don't understand basketball well enough to see the problem, and have made an arms race to try to figure out who can best make predictions using their stat and their stat alone.
While I'm ranting:
RAPM is itself based on APM, and APM did have a universal standard. I wasn't opposed to adding RAPM to supplement APM because the need to reduce noise was large, but it basically replaced APM with common availability and the statistician community never seemed to understand why APM had utility that could not be made obsolete by RAPM.
Last thing:
To be clear, as I alluded to in my post above, I don't think it ever makes sense to only look at regressed +/- when doing a thorough analysis. The raw stuff provides context that allows you to better grasp what the regressed stuff can speak to. But the regressed stuff could be better laid out in the public domain, and if it were, I'd feel more comfortable starting conversations with it.